The present invention relates to a method for reproducing optically an information recorded on a record medium along tracks by means of a light beam, and to an apparatus for carrying out such an optically reproducing method. Applicants have filed two other applications revealing similar subject matter having U.S. patent application Ser. Nos. 293,082 and 301,098, filed on Aug. 14, 1981, and Sept. 10, 1981, respectively.
Such information reproducing method and apparatus have been known and have been advantageously applied to an apparatus in which a scanning light spot is projected by an objective lens onto information tracks formed spirally or concentrically in a disc-shaped record medium to read an information recorded along the tracks.
In an apparatus for reproducing or picking-up an information from the above mentioned record medium, the record medium is usually called as a video disc or audio disc in which encoded video and/or audio signals are recorded as optical information such as optical transmittivity, reflection and phase properties. While the video disc is rotated at a high speed such as thirty revolutions per second, i.e. 1,800 rpm, a laser beam emitted from a laser light source is focussed on the tracks of the disc as a light spot and the optical information is read out by detecting the reflected light beam modulated by the information. One of important properties of such a record medium is a very high density of recorded information and thus a width of the information track is very narrow and a space between successive tracks, i.e. a track pitch is also very narrow. In a typical video disc, a pitch of the tracks amounts only to 2 .mu.m. Therefore, the diameter of light spot should be correspondingly small such as 1 to 2 .mu.m. In order to pick-up correctly the recorded information from such tracks having very narrow width and pitch, an error in a distance between the objective lens and the tracks, i.e. a focussing error should be reduced as little as possible to make a spot diameter as small as possible.
To this end, the apparatus is provided with a focussing control system in which an amount and a direction of a de-focussed condition of the objective lens with respect to the disc surface are detected to produce a focussing error signal and the objective lens is moved in a direction of the optical axis of objective lens in accordance with the detected focussing error signal.
Furthermore, during the reproduction, the light spot should follow the track precisely. For this purpose, the reproducing apparatus is also provided with a tracking control system in which an error in a position of the light spot with respect to the track, i.e. a tracking error is detected to produce a tracking error signal and the light spot is moved in a direction perpendicular to the track, i.e. a radial direction of the disc in accordance with the detected tracking error signal.
FIG. 1 is a schematic view illustrating an optical pick-up apparatus comprising an embodiment of a focus detection apparatus proposed by the applicant. In this embodiment, a light beam (linearly polarized in a plane of the drawing) emitted from a laser light source 1 is collimated into a parallel light beam by a collimator lens 2 and passes through a polarizing prism 3 and a quarter-wavelength plate 4. Then, the parallel light beam impinges upon an objective lens 5 and is focussed on an information track of a disc 6 as a light spot. The light beam is reflected by an information track (record surface) 7 having a crenellated pit construction of the disc 6 and is optically modulated in accordance with information recorded in the track. Then the light beam is reflected by the polarizing prism 3, because the light beam is polarized in a direction perpendicular to the plane of the drawing and thus, it has passed through the quarter-wavelength plate 4 twice. The light flux reflected by the polarization prism 3 impinges upon a detection prism 8 having a reflection surface 9 and the light flux reflected by this surface 9 is received by a light detector 10. The reflection surface 9 is so arranged with respect to the incident light that under an in-focussed condition it makes a given angle with respect to the incident light (parallel light flux) which angle is equal to a critical angle or slightly smaller or greater than the critical angle. Now, for the time being, it is assumed that the reflection surface 9 is set at the critical angle. In the in-focussed condition, the whole light flux reflected by the polarizing prism 3 is totally reflected by the reflection surface 9. In practice, a small amount of light is transmitted into a direction n shown in FIG. 1 due to incompleteness of a surface condition of the reflection surface 9. However, such a small amount of transmitted light may be ignored. If the disc 6 deviates from the in-focussed condition in a direction a in FIG. 1 and a distance between the objective lens 5 and the disc 6 is shortened, the light reflected by the polarizing prism 3 is no longer the parallel beam, but changes into a diverging light beam including extreme light rays ai.sub.1 and ai.sub.2. On the contrary, if the disc 6 deviates in an opposite direction b, the parallel light beam is changed into a converging light beam including extreme light rays bi.sub.1 and bi.sub.2. As can be seen in FIG. 1, light rays from an incident optical axis OP.sub.i to the extreme light ray ai.sub.1 have incident angles smaller than the critical angle and thus, are transmitted through the reflection surface 9 at least partially as illustrated by at.sub.1 (the reflected light being shown by ar.sub.1). Contrary to this, light rays between the optical axis OP.sub.i and the extreme light ray ai.sub.2 have incident angles larger than the critical angle and thus, are totally reflected by the surface 9 as shown by ar.sub.2. In case of deviation of the disc 6 in the direction b, the above relation becomes inversed, and light rays below a plane which includes the incident optical axis OP.sub.i and is perpendicular to the plane of the drawing of FIG. 1, i.e. a plane of incidence, are totally reflected by the reflection surface 9 as denoted by br.sub.1, and light rays above said plane are at least partially transmitted through the reflection surface 9 as depicted by bt.sub.2 (the reflected light being illustrated by br.sub.2). As explained above, if the disc 6 deviates from the in-focussed position, the incident angles of the light rays impinging upon the reflection surface 9 vary in a continuous manner about the critical angle except for the center light ray passing along the optical axis OP.sub.i. Therefore, when the disc 6 deviates from the in-focussed position either in the direction a or b, the intensity of the light reflected by the reflection surface 9 varies abruptly near the critical angle in accordance with the above mentioned variation in the incident angles as illustrated in FIG. 2. In this case, senses of the variations of the light intensities on both sides of said plane perpendicular to the incident plane and including the incident optical axis OP.sub.i vary in mutually opposite manner. On the contrary, in the in-focussed condition, the light flux impinging upon the detection prism 8 is totally reflected by the reflection surface 9 and thus, the uniform light flux impinges upon the light detector 10. The light detector 10 is so constructed that the lower and upper light fluxes with respect to said plane are separately received by separate regions 10A and 10B, respectively. That is to say, the light detector 10 is divided along a plane which is perpendicular to the incident plane and includes an optical axis OP.sub.r of reflected light.
FIG. 2 shows a graph representing a variation of an intensity of reflected light in accordance with an incident angle near the critical angle. Curves R.sub.p and R.sub.s indicate the light intensities for P and S polarized light rays, respectively. The curves are obtained when the detection prism 8 is made of material having a refractive index of 1.50. It should be noted that an intensity of a non-polarized light ray is equal to an intermediate value of (R.sub.p +R.sub.s)/2.
In FIG. 1, if the disc 6 deviates in the direction a, the light rays of the lower half of the incident light flux have incident angles smaller than the critical angle. Therefore, at least a part of the lower half light flux is transmitted through the reflection surface 9 and the amount of light impinging upon the light receiving region 10A is decreased by an amount equal to the transmitted light. While the upper half of the incident light flux has the incident angles larger than the critical angle and thus, is totally reflected by the surface 9. Therefore, the amount of light impinging upon the light receiving region 10B is not changed. On the contrary, if the disc 6 deviates in the direction b, the amount of light impinging upon the region 10B is decreased, but the amount of light impinging upon the region 10A is not changed. In the in-focussed condition, amounts of light impinging upon the regions 10A and 10B are made equal to each other.
It should be noted that the reflection surface 9 may be set at an angle slightly smaller or larger than the critical angle. In a former case when the disc 6 deviates in the direction a, the amount of light impinging upon the region 10B is first increased and then becomes constant and the amount of light impinging upon the region 10A is decreased abruptly. Whereas, if the disc 6 deviates in the direction b, the amount of light impinging upon the region 10A is first increased and then becomes constant, while the amount of light impinging upon the region 10B is decreased.
In this manner by detecting a difference in output signals from the light receiving regions 10A and 10B by means of a differential amplifier 11, it is possible to obtain the focussing error signal having an amplitude which is proportional to an amount of the deviation from the in-focussed condition and a polarity which represents a direction of the deviation with respect to the in-focussed condition. The focussing error signal thus obtained is used to effect a focussing control for driving the objective lens 5 in the direction of its optical axis. Further, it is possible to derive an information signal corresponding to the pit information recorded in the information track 7 at an output of an adder 12 which produces a sum signal of the output signals from the regions 10A and 10B. Further, in the in-focussed condition, since the light is scarcely transmitted through the reflection surface 9, a loss of light is very small and in the defocussed condition the half of light flux with respect to the central light ray is totally reflected, but an amount of the other half of light flux reflected by the surface 9 is decreased to a great extent, the difference in the amount of light impinging upon the regions 10A and 10B becomes great. Therefore, the very accurate focus detection can be effected with a very high sensitivity.
For instance, when use is made of the objective lens 5 having a numerical aperture NA=0.5 and a focal length f=3 mm and of the detection prism 8 having a refractive index n=1.50 and the disc 6 deviates by about 1 .mu.m, a variation of an incident angle for the extreme right ray which is subjected to the largest variation in incident angle is about 0.015.degree. when can cause a sufficiently large variation in light amount impinging upon the detector regions 10A and 10B.
In the apparatus shown in FIG. 1, the light reflected by the reflection surface 9 is received by the detector 10 having the two light receiving regions 10A and 10B, but it is also possible to detect the light transmitted through the reflection surface 9 by the two light receiving regions or to detect both the reflected light and the transmitted light by two separate light detectors.
In case of reproducing the information out of the information record medium such as the video disc, it is not sufficient to effect the above mentioned focussing control so as to project the small light spot on the information record surface, and it is also necessary to effect a tracking control so as to cause the light spot to follow the given information track accurately.
The applicant has proposed a reproducing apparatus for obtaining the focussing error signal, tracking error signal and the information signal by using the focus detection apparatus shown in FIG. 1. In this reproducing apparatus, use is made of a light detector 20 having four light receiving regions 20A, 20B, 20C and 20D divided along two orthogonal directions as shown in FIG. 3. Now it is assumed that these four regions 20A to 20D produce outputs E.sub.A, E.sub.B, E.sub.C and E.sub.D. At first, sum signals E.sub.A +E.sub.D and E.sub.B +E.sub.C are generated by adders 21A and 21B, respectively. Then a difference signal (E.sub.A +E.sub.D)-(E.sub.B +E.sub.C) between these sum signals is formed by a differential amplifier 22 to produce a focussing error signal. At the same time, sum signals E.sub.A +E.sub.C and E.sub.B +E.sub.D are first formed by adders 21C and 21D, respectively. Then a difference signal (E.sub.A +E.sub.C)-(E.sub.B +E.sub.D) is formed by a differential amplifier 23 and a tracking error signal is derived after suitably processing the difference signal thus formed. An information signal is obtained from an adder 24 which forms a sum signal E.sub.A +E.sub.B +E.sub.C +E.sub.D.
In the above mentioned information reproducing apparatus, since the focussing error signal and tracking error signal are detected from the same and single light beam, the tracking error signal is subjected to variation due to the focussing error and the focussing error signal is also affected by the tracking error. Therefore, it is difficult to effect the focussing and tracking servo control in a precise and accurate manner.